Rambles among the Channel Islands / by a Naturalist.

Date:
[1855?]
Catalogue details

Licence: Public Domain Mark

Credit: Rambles among the Channel Islands / by a Naturalist. Source: Wellcome Collection.

    146/402 (page 134)
    being disposed at opposite angles, form spots as though the stone had been sprinkled by a passing shower. Stone of this description is called by the Guernsey masons Talvaen, pro- nounced Talvawne. Associated with the hornblende rock is a rock called greenstone. It is composed of horn- blende and felspar, the latter in a more or less highly crystalline condition. Chlorite occurs united with hornblende, and narrow beds of chlorite schist are also found. This is a green slaty rock, often asociated with, and graduating into gneiss. Gneiss, a rock forming so large and conspi- cuous a portion of these islands, also demands our notice. It is a mineral formed of the same ingredients as granite, but with tliis remarkable difference, that, instead of being arranged, as in the latter, in a crystalline mass, they are ar- ranged in layers; in other words, gneiss is a stratified rock, composed of felspar, quartz, and mica. Its white layers consist almost exclu- sively of granular felspar; the dark layers, of grey quartz and black mica. Many varieties of this mineral are met with. In some, the stratification is perfect; in others, it is scarcely apparent. It is also met with of a red colour. Traversing the gneiss, and often alternating with it, is the rock called trap. This rock